XLSX sheet screenshot omits floating charts and clips to the first populated cell

## Environment

- OfficeCLI: `1.0.138`
- Input: an XLSX worksheet containing populated cells, a pivot table, and a floating chart

## Reproduction

```bash
officecli view workbook.xlsx screenshot --page 1 --range "Dashboard!A1:J20" -o dashboard.png
```

## Actual behavior

The generated PNG is only `254 x 64` and contains the title in `A1`. It omits the populated KPI/pivot range (`A3:B13`) and the floating chart (`D1:J18`).

Additional observations:

- `--page 2` still renders the first worksheet.
- `--range /Dashboard` returns `clip_target_not_found`.
- `--range /Dashboard/chart[1]` renders the chart correctly by itself.
- `officecli view workbook.xlsx html` renders the complete worksheet, including cells, pivot table, and chart.
- `officecli validate` passes and `officecli view workbook.xlsx issues` reports zero issues.

## Expected behavior

A worksheet screenshot should crop to the union of the used cell range and floating drawing/chart bounds, or the CLI should provide an explicit worksheet screenshot selector such as `--sheet`.

## Impact

This does not affect workbook creation, editing, formulas, pivots, or chart data. It prevents a single-command PNG QA/preview of a complete worksheet containing floating charts.

Downstream tracking: YellowDi/LingStack#60

Start by reproducing `officecli view workbook.xlsx screenshot --page 1 --range "Dashboard!A1:J20" -o dashboard.png`, then compare it with the complete `html` rendering and the standalone `/Dashboard/chart[1]` selector. Done means a worksheet screenshot includes both the used cell range and floating chart bounds, or a documented explicit worksheet selector such as `--sheet` is provided.

Written by the indexing model from the issue text.